During questions in the House of Commons today, defence secretary Philip Hammond was asked (not for the first time) when the UK would announce how many British troops will withdraw from Afghanistan in 2013 and 2014.

And (not for the first time) Hammond said that this depended on the United States. The UK will show its hand once President Obama has made up his mind about the American withdrawal.

Obama is unlikely to do that before next May, when there is a conference in Chicago on the future of Afghanistan.

Two questions spring to mind: why are we waiting? And what benefit can we gain? The answers seem to be - nobody really knows, and not very much.

By next summer, the US will still have 68,000 troops left in Afghanistan. Most of them will have to be out by the end of 2014 to meet the time-table for transition - when the country's security will be entirely in the hands of local forces, and Nato's remaining troops will be in a non-combat role.

So Obama's decision is really about the speed of withdrawal over the next three years. He has many factors to consider, and it would seem far-fetched to assume British interests will be at the forefront of his thinking.

This, they argue, will give Nato one more full year of fighting the Taliban with a really substantial force, and allow the Afghan security forces another year to develop before shouldering the responsibility for themselves.

They don't want a drip drip withdrawal in 2013, they want a waterfall in 2014. Whether you agree with that strategy or not, there is no reason to wait until the middle of next year to decide. No reason, that is, unless you are fighting a presidential election.

By next May, Obama will have five months of campaigning left. A cynic might think - what better time to swerve around the advice of his generals and announce a crowd-pleasing withdrawal of troops before the end of the year?

Only President Obama gains from waiting until the middle of next year. In the meantime everyone else must make a variety of contingency plans and wait for the white smoke to rise from the White House chimney.

Nato's withdrawal looks like it is being dictated by an internal political drumbeat, not a military one, or one that asks the most important question - what is best for the future of Afghanistan?

The UK's position in all this is even more perplexing. At a meeting of the National Security Council last week, the Prime Minister was given various potential timetables for British withdrawal from Helmand but, as Hammond reiterated today, the UK will not make its move until Obama has made up his mind.

This is even more illogical when you think that, in some respects, the UK has already - unilaterally - made an important announcement.

The US Marines have been in some of the most violent districts of Helmand, including Sangin. Who will look after these areas when they have gone?Not the Brits, says Hammond. He insists we will not step one inch out of our current area of operations.

"US commanders understand that. People in the White House and Pentagon understand that ... we could not have been clearer in making that point. We are not, and will not extend the area of operations."

Afghan forces will be expected to take the place of the Americans, whether they are ready or not. Other Nato countries may even have to send their troops to Helmand because the UK has drawn a line in the sand.

Right or wrong, the UK has set out its stall. We are already going it alone. Presumably Downing Street, the Ministry of Defence and the Foreign Office, are relying on this being reflected in America's plans for Helmand.

Put simply, Britain doesn't want to be left 'carrying the baby' when the Marines head home. But if the British declaration is supposed to 'inform' US thinking, then it would be better still if the UK went a step further and set out as soon as possible British plans for pulling out of Helmand.

It would give the British battlegroups a chance to plan properly, rather than on the hoof.

But we are not going to do that. We will wait on Uncle Sam. Or rather we will wait on Obama, as he keeps a weather eye on the polls over the next six months.

If the White House agreed with the military advice, Obama could announce a decision tomorrow. He took a bold decision once before, by surging 30,000 extra troops into the country two years ago.

But nobody is expecting him to be so bold again.

The mantra for Nato commanders is that the pull-out will be 'conditions based', but the conditions look like they are increasingly rooted in domestic US politics, and not in the security of Afghanistan.